Effects of a Cretaceous structural inversion and a postulated high heat flow event on petroleum system of the western Lower Saxony Basin and the charge history of the Apeldorn gas field

Abstract

Thermal history and evolution of the Palaeozoic petroleum system of the western Lower Saxony Basin as well as charge history of the Apeldorn gas field was reconstructed using 2-D forward basin modelling software. The Apeldorn gas field is located on an inverted western rim of the Lower Saxony Basin (LSB) and belongs with its relatively anomalous nitrogen content of 73.9 vol. % to one of the most unique gas accumulations in North Germany. Based on thermal calibration studies utilising both, vitrinite reflectance and corrected bottom hole temperatures as calibration parameters, a shallow burial model and an anomalous event of Coniacian high heat flow of 80 to 120 mW/m2 was derived. As result, Lower Triassic and younger successions became subjected to slight changes in thermal maturity as opposed to Carboniferous and Permian successions, which show no assessable impact of the high heat flow event on the coalification pattern. The deep burial model in contrary to the shallow burial model is not supported by the structural reconstruction and backstripping in this more marginal setting. According to the modelling results, the key charge of the present Apeldorn gas field began in Tithonian (late Upper Jurassic) during the major phase of rifting in the Lower Saxony Basin. The present Westphalian coal-derived gas accumulations of the Lower Triassic Buntsandstein reservoir were sourced directly from modelled methane pools at top Rotliegend level. The hydrocarbon potential of the Westphalian source rocks became exhausted in Oxfordian (early Upper Jurassic). Reduction of the hydrostatic pressure during the Coniacian high heat flow event together with uplift during the Coniacian-Santonian inversion led to an extensive free gas exsolution. The resulting gas mixture between the exsolved free gas and the Westphalian coal-derived gas reached and saturated Buntsandstein reservoir. The structural trap became destroyed in course of the inversion leading to a sharp decrease of methane and nitrogen saturation.

Fluid inclusion and microfabric studies on Zechstein carbonates (Ca2) and related fracture mineralizations – New insights on gas migration in the Lower Saxony Basin (Germany)

“…Betz et al (1987) andZiegler (1990) for example stated that intrusions of magmatic bodies below the LSB may have occurred during the Aptian, while other authors showed that inversion started in the Late Cretaceous (SantonianConiacian; e.g. Kus et al, 2005). Actually, CO 2 -migration in our model coincides with proposed ages of plutonic intrusions in the LSB, though no further evidence for a plutonic body below the study area could be found.…”

“…(Bruns et al, 2013;Norden et al, 2008), this increased heat flow appears negligible and may be caused by migration of hydrothermal fluids restricted to the vicinity of faults (hydrothermal convection; Bjørlykke et al, 1988). As demonstrated by Kus et al (2005) for the western part of the LSB, heat flow values of ca. 110 mW/m 2 during the Coniacian would explain thermal maturity at relatively shallow depth of 3000 m. The mismatch of our thermobarometric results and the vitrinite reflectance calibration compared to published heat flow values in adjacent areas (Kus et al, 2005;Zwart, 1995) can be explained by significant differences in basin evolution within the LSB, especially in the western part.…”

“…Normal faults, which had formed during the Late Jurassic -Early Cretaceous rifting phase were reactivated and reversed (Figure 3) (Betz et al, 1987;Lohr et al, 2007;Scheck-Wenderoth and Lamarche, 2005); compressional inversion tectonics were accompanied by sinistral convergent wrench faulting (Betz et al, 1987). The Late Cretaceous inversion not only resulted in M A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPT 9 the structural modification of the LSB (Baldschuhn et al, 1991;Baldschuhn et al, 2001;Kockel, 1994), but also largely affected the role of the fluid Kus et al, 2005;Lüders et al, 2005;Petmecky et al, 1999). When compressive tectonic forces reactivated previously existing normal faults as reverse faults gas migration was initiated (Schmidt Mumm and Wolfgramm, 2004).…”

“…Thermal maturity data suggest that the effects have been different for different parts of the LSB during basin inversion and tectonic uplift, causing heterogeneous heat flow rates (e.g. Adrisola-Muñoz, 2007;Bruns et al, 2013;Kockel et al, 1994;Kus et al, 2005).…”

“…Sedimentation and erosion rates of Cretaceous sediments were fitted using compiled data by Petmecky et al (1999), Adrisola-Muñoz (2007), and Kus et al (2005). Data used for calibrating heat flow variations include simulated data of Adrisola-Muñoz (2007) and of Petmecky et al (1999) for the study area.…”